ssh connections hang; un-hang on subsequent connection

From: Metal Head (schadsey_at_gmail.com)
Date: 07/28/04 

Date: 28 Jul 2004 12:16:06 -0700

I'm experiencing a strange problem with ssh connections to my ssh server
at home. I suspect it might have something to do with either my firewall
or my DSL hardware, but as I'm not certain I thought I'd ask here to
find out if anyone's seen this behavior before.

In a nutshell, I can make an initial connection to my ssh server fine.
The problem is if I make a second connection, and exit, the first
connection will frequently "freeze" and the session will not respond.
When I re-connect with ssh (or sftp), the first session "thaws" and all
the keystrokes I had sent that appeared to not have made it to the server
happen all at once. Then both connections operate normally. But if I
disconnect the second connection, the initial connection again freezes.
I haven't been keeping track of when this occurs, but it's fair to say
that it happens every time I make and disconnect a second ssh connection
to my server. It also occurs when I connect with a second session and
disconnect the first one.

When I re-connect to "thaw" the frozen session, there is no precise point
that I can tell when things return to normal. It seems that all I have
to do is ssh to my server and hit <Ctrl-C> when the passphrase prompts.
The first session will thaw for anywhere from a minute to several minutes
before it freezes again.

I experience this most often at work, when I ssh to home. A friend with
a different ISP half-way around the country has duplicated the problem
connecting to my server too, and I've also duplicated it from home.
I think that narrows it down to a problem with my setup.

The ssh server is SSH-2.0-OpenSSH_3.8.1p1 Debian 1:3.8.1p1-4 on a Linux
box running kernel 2.4.21. My ssh server runs on my firewall box which
uses iptables 1.2.9. Using SSH protocol 2 only. I've experienced the
problem with both OpenSSH_3.8.1p1 and Putty clients.

I've run tcpdump to look at the traffic and as best as I can determine
there seems to be a problem with "TCP Retransmission" packets at about
the same time the session freezes. Interpreting the capture file is
beyond my abilities though, but I can make it available to anyone who
wants to help me figure this out.

Thanks,

- Steve

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